1. Field of the Invention
The present invention relates to polyamic acid copolymer systems which yield a polyimide upon curing and which find particular application in semiconductor manufacturing.
2. Background to the Invention
Typical prior art dealing with overcoating or packaging of electronic devices includes IBM Technical Disclosure Bulletin 16, 728(1973) Cook et al and U.S. Pat. No. 4,001,870 Saiki et al, both hereby incorporated by reference.
U.S. Pat. No. 4,182,853 Kruh discloses polyamideimide materials containing at least one tricarboxylic monoanhydride, a material alien to the present invention. Polyamideimides are prepared via a polyamideimide polyamine intermediate and a tetracarboxylic dianhydride, forming a copolymer.
U.S. Pat. No. 3,640,969 Suzuki et al discloses the preparation of a soluble polyimide resin and processing without preparing a polyamic acid precursor. While this patent does note the storage problem due to the instability of polyamic acids, the copolymer produced by the disclosed method is mainly mellophanic dianhydride (50-85%), a material alien to the present invention. An imidization agent is also used in this process.
U.S. Pat. No. 3,983,093 Williams et al discloses polyetherimides derived from an organic diamine, a bisphenol dianhydride and pyromellitic anhydride. The recited bisphenol dianhydrides do not include oxydiphthalic dianhydrides and, in fact, always contain two or more anhydride-ether linkages, not one as in oxydiphthalic dianhydride.
U.S. Pat. No. 3,832,330 Dixon et al discloses anhydride and urethane or urea derivatives of aromatic diamines and prepolymers obtained as intermediates for preparing polyimides. Dianhydrides and urea derivatives (or urethanes) are melted to form a prepolymer at 180.degree.-200.degree. C. with urethanes or at 120.degree. C. with ureas, eliminating CO.sub.2 and producing an alcohol.
U.S. Pat. No. 4,073,788 Peterson discloses a process for producing polyamic acid polymers via novel intermediate reaction products utilizing an imidized polymer to control viscosity of a polyamic acid-imide at the time of application. Stable precursor oligomers are described which continue to polymerize or "zip up" when the stoichiometry of the oligomer is balanced. The precursor oligomers are preimidized by heating the oligomer solution and subsequent addition of unimidized precursors or mixed copolymer precursors is described. If desired, an amine can be added to the prepolymer oligomer to form an ammonium polyamate salt or polyimide salt which is a water soluble prepolymer that can be used as an aqueous-organic solvent coating medium.
U.S. Pat. No. 4,214,071 Alvino et al discloses amide-imide polymers formed from novel amines which contain at least one amide linkage. Mention is made of polyamic acid viscosity stability and oxydiphthalic dianhydride is mentioned. Solutions of the prepared amide-imide polymers are stated to be "stable" for long periods. It is preferred that the novel amines be prepared through reduction of a nitroimide precursor derived from a nitroaniline and an aromatic acylanhydride. A film of the disclosed polymers will adhere to cured Kapton when cured at 250.degree. C. and then laminated at 275.degree. C. at 250 psi for 15 minutes.
U.S. Pat. No. 4,220,750 Reinhardt et al discloses polyimide thermoplastics with enyne functionality that provide cross-linking for reinforced structural composites. The materials are described as having the advantage of a low Tg which increases on thermal processing; control of Tg by copolymerization and variation of Tg on thermal cycling is disclosed.
U.S. Pat. No. 4,269,968 Duran et al discloses a method of polyamic acid synthesis using compacted dianhydride materials.
U.S. Pat. No. 4,302,575 Takekoshi discloses polyetheramide acids and polyetherimides having terminal aliphatic unsaturated groups which are heat curable; these materials are derived from bis(etheranhydrides) and diamines, aliphatic unsaturated dianhydrides being used as end caps. Such materials are stated to have improved processability in the temperature range of 150.degree.-190.degree. C.
U.S. Pat. Nos. 3,179,634 Edwards and 3,264,250 Gall both assigned to Dupont disclose polyimides including pyromellitic dianhydride-benzophenone tetracarboxylic acid dianhydride-oxydianiline copolymers.
In the following discussion, the following abbreviations are often used:
Tg is glass transition temperature. PA1 PMDA is pyromellitic dianhydride. PA1 ODPA is oxydiphthalic dianhydride. PA1 ODA is oxydianiline.
It is known that polyimides formed by reacting PMDA and ODA to polyamic acid and then thermally curing the same to yield the desired polyimide can be used as a passivation dielectric on, e.g., SAMOS devices. See, for example, IBM J. of Research and Development 24, 268-282(1980). The copolymer systems of the present invention could be used in the devices disclosed in these two publications and also in combination with conventional metallized ceramic substrates.
Polyimides of the above type are characterized by good dielectric properties, thermal stability and improved strength on heating. However, such polyimides show a poor ability to adhere to themselves or to metallized surfaces.
Another problem with such polyimides is that they exhibit unstable viscosity characteristics in the form of a polyamic acid solution, complicating application to desired electronic device surfaces, e.g., wafer surfaces.
Further, a thermally cured PMDA-ODA polymer does not have a well defined Tg.